This invention relates to digital processing apparatus for sensors and more particularly to a digital interface apparatus which provides output amplification and enhancement for sensing elements which operate in a certain frequency mode such as planar coil position sensors.
Planar coils have been described in various patents which have been assigned to the assignee herein and which patents describe planar coil configurations which are non-contacting sensors. See for example U.S. Pat. No. 4,507,638 which issued on Mar. 26 1985 to Amnon Brosh, a co-inventor herein, and entitled ROTARY POSITION SENSORS EMPLOYING PLANAR COILS.
In the above noted patent there is described in THE BACKGROUND OF THE INVENTION various other patents which pertain to the planar coil sensors to be described herein. The techniques to be described have broader applications to other types of sensors. It is a purpose of this invention to achieve amplification of sensor output without utilizing typical amplifier circuits which exist in the prior art. Many such sensors have their outputs amplified by operational amplifiers or other linear circuits.
There are, of course, certain obvious advantages which will be achieved by this invention for obtaining amplification using digital techniques as compared to amplification using analog techniques. As is known in the prior art, the amplifier is a relatively critical component. For example, such amplifiers are associated with thermal errors which affect the offset and the gain performance of the amplifier and which errors contribute to overall sensor errors. Furthermore, in order to use operational amplifiers and other linear amplification circuits, several precision resistors have to be used in conjunction with the IC's Such amplifiers are also associated with EMI susceptability which results in output errors.
Generally, linear circuits such as operational amplifiers employ a different and more costly manufacturing technology than digital circuits. Digital circuits which consist of various digital components such as gates, flip flops, counters and other similar devices are less costly to manufacture. The purpose of this invention is to use digital techniques exclusively to drive, to detect and to amplify the sensor output. As is well known, by using digital techniques exclusively, one can achieve a custom integrated circuit made by gate arrays or custom chip techniques at a much more economical cost and by simpler procedures than linear circuits or linear-digital circuit combinations.
Hence it is the object of the present invention to achieve amplification of sensor output while eliminating operational amplifiers or other linear circuits.
It is a further object of this invention to provide a digital circuit which processes the sensor output to provide an amplified signal indicative of the position of a movable member as coupled to a planar coil sensor assembly.
As will be explained, these objects are achieved by employing a digital circuit whereby performance enhancement is also attained due to the fact that the sensor circuit output is a ratio of the difference between two frequency terms divided by their sum. This causes first order thermal and stability errors to cancel out and also results in the cancellation of second order non-linearity errors. The output of the digital circuit is ratiometric with respect to the supply voltage, a feature which is critical when ratiometric analog-to-digital convertors are used in conjunction with the sensor.
Furthermore, by using the circuitry to be described, offset and span adjustment is implemented by utilizing digital techniques whereby adjustment is provided by employing binary numbers. This feature is highly desirable in sensors and sensor interface circuits since prior art adjustments are based on trimming resistors techniques on line which are costly and expensive.